In an Orthogonal Frequency-Division Multiplexing (OFDM) system, each OFDM symbol includes two parts: a Cyclic Prefix (CP) and a useful symbol part. The CP is usually a repetition of a last portion of data in the useful symbol part. In order to apply to different channel environments, a CP length of an OFDM technology-based communication system usually has multiple options.
For example, in a Long Term Evolution (LTE) system adopting an OFDM technology, two types of CPs are supported. For an application scenario of ultra-long-range coverage or multicast information transmission or broadcast information transmission, the system will select a long-CP symbol. At this time, a slot (0.5 ms) includes 6 OFDM symbols, and a CP length of each OFDM symbol is 16.67 us.
For other applications, a communication system will select a short-CP symbol. At this time, a slot includes 7 OFDM symbols, a CP length of the first OFDM symbol of the slot is 5.21 us, and a CP length of other OFDM symbols is 4.69 us.
When a receiver initially accesses an OFDM system, it generally receives a sent signal (commonly referred to as a synchronization signal) to obtain initial synchronization (including OFDM symbol synchronization, frame synchronization, frequency offset correction, cell number identification or the like) with the system. However, the diversity of the aforementioned CP lengths brings a lot of trouble to the synchronization process of the receiver. The receiver has not yet got in touch with the OFDM system and naturally will not know the type of the CP length selected by the OFDM system. For this, there are three solutions as follows.
Solution 1: The receiver tries according to all possible CP types, which will cause exponential increase of the complexity of the synchronization process of the receiver, and is very unfavorable for reducing the power consumption and cost of the receiver. This is especially obvious for a machine type communication receiver.
Solution 2: A synchronization signal is always sent by using a certain agreed CP type. Although this solution reduces the complexity of the synchronization process, it places great restrictions on the scheduling of the system. For example, the aforementioned Narrow Band (NB)-LTE system and LTE system coexist in the same frequency spectrum. For example, in an LTE system with a system bandwidth of 20 MHz, a bandwidth of 180 kHz is allocated for sending signals of an NB-LTE system. However, due to the difference between service receivers of the NB-LTE system and the LTE system, the requirements for CP types are also different. If the CP type of the LTE system is forced to be consistent with the CP type, for sending a synchronization signal, of the NB-LTE system, it inevitably leads to a significant drop in the efficiency of the LTE system.
Solution 3: A synchronization signal is always sent on a last symbol in a slot. This method is a method employed by an existing LTE system (in an LTE system, a Primary Synchronization Signal (PSS) for symbol timing is on a last OFDM symbol in a slot). At the same time, the defects of the above two solutions are avoided. However, this method requires that a synchronization signal must be sent on only one OFDM symbol in a slot. For the aforementioned NB-LTE system, the bandwidth thereof may be only 180 kHz. If it is defined that the synchronization signal can be sent on only one OFDM symbol in a slot, the energy of a synchronization signal received by the receiver for a single time is very limited, and the receiver often needs to accumulatively receive synchronization signals of a plurality of slots to realize synchronization with the system. Compared with the existing LTE system, this will significantly increase the power consumption and synchronization time of the receiver, and also reduce the efficiency of the aforementioned narrowband system.
In the above solutions, in order to increase receiving performance of a synchronization signal, in some cases, synchronization signal will be sent on consecutive OFDM symbols. Because CP lengths of these OFDM symbols are different, the implementation of signal detection and synchronization processing in the synchronization process is more complicated, more processing resources and power are consumed, and the problem becomes more prominent.